feat: skip over unknown directives

[3-w-c]

Fixes #14, #15

This is my first contribution to anything RST, so it might not be the best approach 😅

Test file:

######################
 Prior research
######################

According to :cite:t:`nutt2009`

Before this PR:

docutils.utils.SystemMessage: :16: (ERROR/3) Unknown interpreted text role "cite:t".

After this PR:

• :cite:p: kept as-is 🥳

Future enhancement could be to output warnings, maybe?